This invention relates to a semi-transparent multi-layer film structure that has a metallized surface. The bonding of metals, such as, aluminum, silver, chromium, etc., to plastic films and the gas barrier developed, has allowed such films to replace metallic foils in many instances. The flexibility of the films necessitates the formation of a strong metal/plastic bond and a number of approaches have been developed for providing such bonding. In some cases a special primer layer must be applied to the base layer in order to achieve an effective bond between the metal and the substrate. In many cases a thermoplastic surface must be corona discharged treated in order for there to be an effective bond between the metal and the thermoplastic surface. Gas barrier properties will also depend upon the condition of the surface on which the metal is deposited.
It is known in the art to enhance the metallization performance of polypropylene-based films by using a polymer other than homopolymer polypropylene. U.S. Pat. No. 4,345,004 to All et al. discloses a metallizable skin made of an ethylene-propylene copolymer. U.S. Pat. No. 4,741,957 to Park describes a skin layer made of polyamide, polyester, or polycarbonate polymers which provides improved surface properties for lamination and printing. Japanese Patent 55-126057 discloses the use of a 0.60 mil EVOH film with no elongation for metal deposition. Japanese Patent No. 1-267032 discloses 0.20 to 16 mil EVOH film on which is deposited a silicon oxide, which film can be heat treated to enhance oxygen barrier and water resistance. Generally such films provide enhanced barrier properties provided that the films are metallized at a critical thickness, i.e., an optical density of greater than 2, or a metal thickness of 10 to 100 nm.
U.S. Pat. No. 5,153,074 to Migliorini discloses a metallized film combination comprising a propylene polymer substrate modified with maleic anhydride, a skin layer of EVOH metallized with an aluminum coating of 100 to 500 angstroms (10 to 50 nm) to provide a film having an optical density of 1.5 to 3.0, and an optional heat sealable skin layer.
It is an object of the present invention to provide a semi-transparent, heat sealable metallized film having moderate light/UV and microwave transparency which nonetheless exhibits excellent oxygen gas barrier and water vapor barrier.
In accordance with the present invention an oriented multi-layer film combination comprises:
(a) a core layer of polypropylene homopolymer or copolymer;
(b) a metallizable skin layer comprising a member selected from the group consisting of ethylene-vinyl alcohol copolymer (EVOH), poly/vinyl alcohol) (PVOH), and polyester, adjacent one surface of said core layer, said layer being metallized to a metal thickness no greater than 5 nm; and
(c) adhesion promoting amounts of an adhesive selected from the group consisting of maleic acid anhydride modified polypropylene homopolymer or copolymer, high density polyethylene, and ethylene-vinyl acetate (EVA) copolymer, disposed between (a) and (b). In one embodiment of the invention, the multilayer film combination further comprises (d) a heat sealable layer disposed on the other surface of said core layer.
This film combination presents a high surface energy surface to which can be strongly bonded a very thin metallized layer, e.g., a thickness of no more than 5 nm, to provide a metallized film of optical density less than 1.5, preferably no greater than 1.
The present invention also relates to a method of forming a metallized multi-film layer combination which comprises
(a) coextruding i) a core layer of polypropylene homopolymer or copolymer, ii) a metallizable skin layer comprising a member selected from the group consisting of ethylene-vinyl alcohol copolymer (EVOH), poly(vinyl alcohol) (PVOH), and polyester, adjacent one surface of said core layer; wherein adhesion promoting amounts of an adhesive selected from the group consisting of maleic acid anhydride modified polypropylene homopolymer or copolymer, high density polyethylene, and ethylene-vinyl acetate (EVA) copolymer are disposed between (i) and (ii);
(b) quenching the coextruded product of (a) on a casting drum in a water bath to form a base sheet having a thickness of 120 to 1600 mil;
(c) orienting the base sheet in the machine direction by stretching 4 to 8 times, and in the transverse direction by stretching 5 to 12 times to form an oriented film having a thickness of 0.5 to 2.0 mil;
(d) metallizing the metallizable skin layer to provide a layer of deposited metal of no greater than 5 nm.
In one embodiment, the coextruding is further carried out with iii) a heat sealable layer disposed on the other surface of said core layer.
The core substrates contemplated herein include the homopolymers and copolymers of polypropylene, particularly preferred are isotactic polypropylenes containing at least 80% by weight of isotactic polypropylene. This material has a density ranging from about 0.90 to 0.91, a crystalline melting point of about 160-165xc2x0 C., and a melt index of about 2-6, preferably 3-4, grams/10 minutes. This material is about 80-100% isotactic, and preferably 95-96% isotactic. Commercially available materials of this description are Exxon 4252, ARCO W472, and Fina 3371. Suitable core polypropylene materials are further disclosed in U.S. Pat. No. 4,888,237 to Balloni et al which is incorporated herein by reference in its entirety.
The surface of the polypropylene or polypropylene copolymer is modified by a particular functionalized adhesive material which serves to tie the polypropylene substrate and the metallizable layer together. This adhesive can be mixed into or applied on top of the polypropylene substrate, so that the metallizable skin layer will aggressively adhere to this so-modified material. If a maleic anhydride modified polypropylene homopolymer or copolymer becomes a part of the substrate surface either by being interblended therewith or by being extruded thereon as a surface skin layer, this then becomes an ideal surface for receiving the metallizable skin layer of the present invention. This adhesive modified surface layer or adhesive skin layer itself cannot be separated from the propylene homopolymer or copolymer substrate. By a xe2x80x9cmaleic anhydride modified polypropylene homopolymer or copolymerxe2x80x9d is meant the product resulting from the reaction between maleic anhydride and the thermal degradation product of polypropylene or polypropylene copolymer. Examples of this material can be found disclosed in U.S. Pat. No. 3,480,580 issued Nov. 25, 1969, the disclosure of which is incorporated herein by reference in its entirety. Particular attention is directed to examples 3, 4 and 6 of the specification.
The maleic anhydride modified polypropylene or a copolymer thereof can be prepared by a fairly simple process. For example, about 125 grams of low viscosity polypropylene of 3300 centipoise at 190xc2x0 C. and 7.5 grams of maleic anhydride are placed in a 500 ml glass round-bottom flask. This flask has three necks and is fitted with a metal sweeper, a dropping funnel and a steam-jacketed condenser to return maleic anhydride which boils out of the reaction mixture. The flask is lowered into a Wood""s metal bath at a temperature of about 200xc2x0 C. After about 1 hour the product is poured from the flask onto a metal tray. The waxy product will have a melt viscosity of 3600 centipoise at 190xc2x0 C. and a saponification number of 16.
Low viscosity polymers can also be produced in the above manner from low molecular weight 98-96/2-4 propylene/ethylene copolymers, 50/50 propylene/butene-1 copolymer, 95/5 propylene/pentene-1 copolymer, 90/10 propylene/hexene-1 copolymer, 80/20 propylene/4-methyl-1-pentene copolymer, 75/25 propylene, dodecene-1 copolymer and 90/10 butene-1/hexene copolymer. In general, propylene copolymerized with another alpha olefin, including ethylene. The procedure for the use of a maleic anhydride modified polypropylene homopolymer or copolymer, when this material is interblended into the base layer is exemplified by the following technique. A maleic anhydride modified polypropylene, e.g., Epolene E-43, a maleic anhydride modified polypropylene obtained from Eastman Kodak Company, melt-blended with a polypropylene homopolymer, such as, Exxon 4252 (85 parts) to give a 15% master batch. 20 parts of the master batch is dry blended with 80 parts of the polypropylene copolymer to give a resin blend containing 3% of the maleic anhydride modified polypropylene. This blend will contain sufficient maleic anhydride-modified polypropylene at the surface of the substrate to aggressively bond the polyamide thereto. The alternative method of making available the maleic anhydride-modified polypropylene homopolymer or copolymer at the surface of the substrate is to coextrude the base polymer with a skin layer containing the maleic anhydride-modified polymer.
Additional materials suitable for use as adhesives are high density polyethylene (HDPE) such as Quantum Plexar 201, available from Quantum Chemical Corp., of Cincinnati, Ohio, and ethylene-vinyl acetate copolymer, e.g., Bynel CXA, available from DuPont Co., Wilmington, Del.
The contemplated metallizable skin layers consist essentially of a member selected from the group consisting of ethylene-vinyl alcohol copolymer (EVOH), poly(vinyl alcohol) (PVOH), and polyester. The EVOH resins useful in this invention include resins having a copolymerized ethylene content of about 20 to 80 mole %, especially about 25 to 50 mole %. Copolymers of lower than about 15 to 20 mole % ethylene, tend to be difficult to extrude while those above 60 or 65 mole % ethylene have reduced oxygen barrier performance. These polymers will have a saponification degree of at least 50%, preferably at least 90%, especially at least 95%. A degree of saponification of less than 90% results in inferior oxygen barrier properties. The ethylene vinyl alcohol copolymer may include as an optional comonomer other olefins such as propylene, butene-1, pentene-1, or 4-methylpentene-1 in such an amount as to not change the inherent properties of the copolymer, that is, usually in an amount of up to about 5 mole % based on the total copolymer. The melting points of these ethylene vinyl alcohol polymers are generally between 150 and 190xc2x0 C.
Ethylene vinyl alcohol polymers are normally prepared by copolymerization of ethylene with vinyl acetate, followed by hydrolysis of the vinyl acetate component to give the vinyl alcohol group. This process is well known in the art. The PVOH polymer used can be a polyvinyl alcohol homopolymer or copolymer, e.g. Vinol 325 (a 98% hydrolyzed medium viscosity polyvinyl alcohol, available from Air Products and Chemicals, Inc. of Allentown, Pa., or ELVANOL 71-30, available from DuPont Co., Wilmington Del.
Suitable polyesters employed for the metallizable skin layers include polyethylene terephthalate (PET). PET is the product of a condensation reaction between ethylene glycol and ethylene terephthalic acid. Such PETs include Petlon, available from Mobay Corp., Pittsburgh, Pa.
As indicated above the metallizable skin layer comprises the surface for receipt of a vacuum metallization. A further advantage of employing such materials is that it is not necessary to corona discharge treat the metallizable surface layer prior to metallization because it inherently has a wetting tension of at least 36 dynes/cm, as determined by ASTM 2578-67 (Dyne Solution Test). Higher wetting tensions are, however, obtainable by employing corona or flame treatment processes. The inherent polar nature of the metallizable skin results in metal adhesion levels far exceeding those obtainable with a treated polypropylene homopolymer skin. The metallized OPP film also exhibits excellent bond strength in both adhesive and extrusion lamination. Further desirable properties of the base film for metallization, are that it displays an excellent oxygen gas barrier when metallized as well as optical clarity and metal uniformity. It also retains the other desirable properties of metallized oriented polypropylene film, such as, good water vapor barrier.
Metallization is carried out in accordance with vacuum metallization procedures using metals such as aluminum, zinc, copper (and alloys thereof such as bronze), silver, gold, and the like, aluminum being preferred for its economy, especially where packaging applications are concerned. A suitable metallizing process consists of placing a roll of film in a vacuum chamber at a pressure of 1xc3x9710xe2x88x923 to 1xc3x9710xe2x88x926 torr. The film is then unwound and travels above a cloud of aluminum, where condensation occurs to produce a very thin aluminum coating no greater than 5 nm in thickness, e.g., from 0.5 to 5 nm. The aluminum cloud is produced by feeding aluminum wire on resistance heated intermetallic boats. The metallized film is then rewound, and removed from the chamber.
It is to be understood that opposite the metallizable surface, i.e., on the other side of the core, the multi-layer film of the present invention has a heat-seal layer applied thereto. Preferred heat-seal layers include ethylene-propylene random copolymer, and an ethylene-propylene-butene-1 terpolymer, ionomer, e.g., Surlyn, available from Dupont Co. of Wilmington, Del., etc., and copolyester, e.g., PCTA, PETG, PCT, and PCTG. Such copolyester materials are available from M.A. Industries, Inc., Peachtree City, Ga. Suitable polyolefin sealants, and their combination with polypropylene cores are further disclosed in U.S. Pat. No. 4,888,237, incorporated herein by reference.